Not enough mains voltage for amp?

ok, this one is tough to explain. One of the venues my pland plays live at is really small, and i mean really small - thirty people is packed to capacity! anyhow, where we set up in this bar has no mains outlet power, so we have to run an extension cable all the way in behind the bar and power everything off of this one socket. so thats two guitar amps, two effects boards, one bass amp and one P.A. system. I am running a marshall TSL100 and my buddy is using a P.O.S. 120W Laney transistor amp. my problem is that i cant seem to get my correct sound on my amp whereas he has no problem, possibly due to the fact that his distortion comes from the efeects board an not from the tubes - could this be due to the fact that we are overloading the hell out of the mains outlet? have never had this problem when we play anywhere else.

the amp is approx two years old and still have the factory fitted tubes. I got it second hand but its in as new condition. the guy i got it from had only ever used it in his bedroom, he was selling it cause it was way more power than he needed.

no, the sound is sweet whenever we play anywhere else as there is usually ample power points for us to plug into, so im pretty confident the tubes have nothing do do with it. the only difference between our setup from venue to venue is the fact that this place has us all plugged into the one power point.